In a manufacturing process of an electronic device such as a semiconductor device, a mask is formed on a target layer, and etching is performed for transferring a pattern of the mask to the target layer. As the mask, a resist mask is generally used. The resist mask is formed by a photolithography technology. Accordingly, a critical dimension of a pattern formed on a layer to be etched is influenced by, for example, a resolution limit of the resist mask formed by the photolithography technology and a pattern density. However, recently, with the high integration of electronic devices, it is required to form a pattern having dimensions smaller than the resolution limit of the resist mask. Therefore, as described in Japanese Patent Application Laid-Open No. 2004-080033, there has been suggested a technology for decreasing a width of an opening provided by a resist mask by forming a silicon oxide film on the resist mask and adjusting the dimensions of the resist mask.
In a method of forming a micro-pattern disclosed in Japanese Patent Application Laid-Open No. 2004-080033, a photoresist pattern is formed on a material film on which a micro pattern is to be formed, and then a silicon oxide film is deposited on the material film, but the silicon oxide film needs to be conformally thinly formed without damaging the photoresist pattern thereunder. Then, dry etching is also performed on a lower film, but at an initial stage, a spacer is formed on a lateral wall of the photoresist pattern and subsequently a polymer film is formed on the photoresist pattern.